Selective gel system for permeability profile control

ABSTRACT

An aqueous gel composition is formed from xanthan gum crosslinked with ions of a transitional metal, resorcinol, and formaldehyde. The gel which initially forms is injected into a formation where it selectively enters pores in a more permeable zone. Once in the more permeable zone, the gel reheals and forms a shear and thermally stable gel. This gel can be used in high temperature formations in addition to those formations having a pH of from about 3.0 to about 10. After the shear and thermally stable gel has formed, a steam-flooding, water-flooding, or a carbon dioxide oil recovery process is commenced in a zone of lesser permeability.

FIELD OF THE INVENTION

This invention relates to a process for recovering oil from asubterranean oil-containing formation. More particularly, this inventionrelates to a method of recovering oil wherein zones of varyingpermeabilities are treated with a selective gel which is thermally andshear stable.

BACKGROUND OF THE INVENTION

When hydrocarbon producing wells are drilled, initial hydrocarbonproduction is usually attained by natural drive mechanisms (water drive,solution gas, or gas cap, e.g.) which force the hydrocarbons into theproducing wellbores. If a hydrocarbon reservoir lacks sufficient porepressure (as imparted by natural drive), to allow naturalpressure-driven production, artificial lift methods (pump or gas lift,e.g.) are used to produce the hydrocarbon.

As a large part of the reservoir energy may be spent during the initial(or "primary" production, it is frequently necessary to use secondaryhydrocarbon production methods to produce the large quantities ofhydrocarbons remaining in the reservoir. Waterflooding is a widespreadtechnique for recovering additional hydrocarbon and usually involves anentire oil or gas field. Water is injected through certain injectionwells selected based on a desired flood pattern and on lithology andgeological deposition of the pay interval. Displaced oil is thenproduced into producing wells in the field.

Advancements in secondary hydrocarbon producing technology has led toseveral improvements in waterflood techniques. For example, theviscosity of the injected water can be increased using certain polymerviscosifiers (such as polyacrylamides, polysaccharides, and biopolymers)to improve the "sweep efficiency" of the injected fluid. This results ingreater displacement of hydrocarbons from the reservoir.

Ability to displace oil from all the producing intervals in ahydrocarbon reservoir is limited by the lithological stratification ofthe reservoir. That is, there are variations in permeability which allowthe higher permeability zones to be swept with injected fluid first andleave a major part of the hydrocarbon saturation in the lowerpermeability intervals in place. Continued injection of flooding fluidresults in "breakthrough" at the producing wells at the highpermeability intervals which renders continued injection of the floodingmedium uneconomical.

Profile control has been used to prevent or correct "breakthrough" athigh permeability intervals. Profile control involves blocking off thehigher permeability intervals in a mature flood so that the floodingmedia is diverted to lower permeability intervals. A gel treatment canbe used to reduce the permeability of a higher interval or zone andthereby improve the sweep efficiency. The treatment must be selective,otherwise, it will not be effective and may even be damaging ifunderswept zones are plugged. To avoid this, a mechanical packer can beused to treat each strata separately but this procedure is expensive andrather tedious.

Another approach is to use polymer gels having selective penetrationproperties which will preferably enter the high permeability zones.However, such a gel is very rare. Chromium crosslinked xanthan gum isselective, but it is limited to reservoirs with a maximum temperature of140° F.

Another selective gel system is chromium crosslinked aminoresinstabilized xanthan gum disclosed in the U.S. Pat. No. 4,716,966. Thispatent is hereby incorporated by reference herein. This system isthermally stable up to 210° F. No known gel systems are both selectiveand stable at temperatures higher than 120° F. Resorcinol andformaldehyde are known to form brittle gels at a pH equal to or greaterthan 9 but such gels are brittle and lack selectivity. When the pH isabout 9 or less a solid gel does not form. If a gel were to form itwould be of poor quality.

Therefore, what is needed is a gel that can selectively enter pores in aformation's zone of greater permeability and subsequently reheal to forma shear and thermally stable gel where the temperature is greater thanabout 210° F. in a pH environment of from about 3 to about 10.

SUMMARY OF THE INVENTION

This invention is directed to an aqueous gellable composition and amethod for using the composition to close pores in a formation's zone ofgreater permeability. The gellable composition comprises water, axanthan gum, transitional metal ions, resorcinal, and formaldehyde.Polyvalent metal ions crosslink with the xanthan gum to form ashearable, selective gel which can be injected into a zone of higherpermeability. Once the gel has entered the formation, it reheals. Heatfrom the formation causes the resorcinol and formaldehyde to crosslinkwith the xanthan gum and form a firm, solid gel which is thermallystable and shear resistant.

After the zone of higher permeability has been blocked by the gel, anenhanced oil recovery method is used to obtain hydrocarbonaceous fluidsfrom a zone of lesser permeability. A variety of enhanced oil recoverymethods can be utilized since the solid gel can withstand temperaturesup to about 250° F. and endure formation conditions where the pH variesfrom about 3.0 to about 10.0 and the salinity can be as high as twentytwo percent total dissolved solids.

It is therefore an object of this invention to provide for amulti-purpose gel which can selectively enter a formation's zone ofgreater permeability, reheal and thereafter form a solid gel which isboth shear and thermally stable.

It is yet another object of this invention to provide for a resorcinoland formaldehyde crosslinked gel which can be used in subterraneanformations where the pH varies from about 3.0 to about 10.0.

It is yet still another object of this invention to provide for axanthan gum crosslinked with chromic ions, resorcinol, and formaldehydeso as to form a solid gel able to withstand formation temperatures up toabout 250° F. and up to about twenty two percent salinity.

It is a still further object of this invention to provide for xanthangel which will form within about 4-12 hours at room temperature with atypical xanthan/chromic ion crosslinked gel consistency which can beinjected into a formation and form a final gel in-situ at highertemperatures.

PREFERRED EMBODIMENTS

In the practice of this invention, a gellable composition is made bymixing into water a water soluble xanthan polymer. Water which issuitable for use herein includes sea water, brackish water, fresh water,and mixtures thereof. Depending upon the formation's characteristics,xanthan gum is mixed into the aqueous solution in an amount of fromabout 0.1 to about 1.0 wt. %, preferably about 0.2 to about 0.5 wt. %.

Redox systems such as sodium dichromate and sodium bisulfite have beenutilized to obtain Cr⁺³ ions. Similar redox systems are described inU.S. Pat. No. 3,749,172 which is hereby incorporated by reference. Whenforming these gels, Cr⁺³ ions are used in an amount of from about 20 toabout 1,000 ppm, preferably about 50 to about 300 ppm. Water solublecr⁺³ salts such as chromic chloride, nitrate, sulfate, and organiccomplexed chromic salts used in leather tanning can also be useddirectly.

As is understood by those skilled in the art, the amount of cr⁺³ ions,or other transition metal ions, utilized will vary depending upon theconcentration and molecular weight of the particular polymer utilized.

A suitable amount of a phenolic compound is next added to the mixture.The phenolic compound utilized can comprise any suitablewater-dispersible phenol or naphthol. Suitable phenols includemonohydroxy and polyhydroxy naphthols. Phenolic compounds suitable foruse in the present invention include phenol, catechol, resorcinol,phloroglucinol, phyrogallol, 4,4'-diphenol, 1,3-dihydroxynaphthalene,and the like. Other phenolic components that can be used include atleast one member of selected oxidized phenolic materials of natural orsynthetic origin such as 1,4-benzoquinone; hydroquinone or quinhydrone;as well as a natural or modified tannin such as quebracho orsulfomethylated quebracho. Resorcinol is the phenolic compound preferredfor use herein in an amount of from about 0.25 to about 5.0 wt. %,preferably about 0.5 to about 3.0 wt. %. Other phenolic compounds can beemployed in an amount comparable to that in molar concentration.

In the next step, an aldehyde is added to the mixture. Aldehydes whichcan be used herein include formaldehyde, polyoxymethylene, tetraoxane,trioxane, and dialdehydes. Dialdehydes which can be used includemalonaldehyde, glutaldehyde, and other water soluble homologs. Aldehydeand dialdehyde should be included in the mixture in about 0.25 to about5.0 wt. %. Formaldehyde is used in the mixture in about 0.5 to about 3.0wt. %.

After the polymer has been mixed in the water, transition metal ions areadded into the mixture. Suitable crosslinking agents include polyvalentmetal cations such as Al⁺³, Cr⁺³, Fe⁺³, Sb⁺³ and Zr⁺⁴. For example,aluminum citrate can be admixed into the mixture. Soluble compounds ofCr⁺³ or Fe⁺³ can be used, or oxidizable compounds of divalent iron suchas FeCl₂ can be used in conjunction with an oxidant. Cr⁺³ ions areparticularly preferred. Chromic nitrate and chromic chloride can also beutilized. The mixing sequence is not limited to this order and saidsequence can be reversed.

After mixing the components together, the gel time, viscosity, andstrength can be controlled to the requirements of a reservoir andexisting field conditions by the addition of a suitable acid or base.Any common inorganic acid such as sulfuric or hydrochloric acid can beused. Bases which can be utilized herein include the hydroxides ofalkali metals such as potassium hydroxide, sodium hydroxide and calciumhydroxide. These and other bases are added to the mixture in an amountup to three times the concentration of the transitional metal ionsemployed in moles.

An initial gel if formed within about 4 to about 12 hours at roomtemperatures. The gel which is formed has a typical xanthan-chromiumconsistency. This initial gel is selective a more permeable zone of aformation. Once the injected gel has remained in the formation for atime sufficient to heat it, it forms a solid, heat and shear stable gel.

A preferred composition of the gel is mixed into 3.5-22 wt % syntheticseawater and includes the following components: (a) xanthan--0.2 toabout 0.5 wt. %; (b) chromium (III) as Cr⁺³ --about 100 ppm by weight;(c) resorcinol--2.5 to about 1.0 wt. %; and (d) formaldehyde--about 0.5wt. %. To this mixture is added a sufficient amount of acid or an alkalimetal hydroxide or other base to obtain a desired gel time and gelstrength.

The following Table illustrates the gel composition and gel time.

TABLE OF EXAMPLES

                                      TABLE OF EXAMPLES                           __________________________________________________________________________       Xanthan.sup.1                                                                      Cr(III)                                                                           Resorcinol                                                                          Formaldehyde                                                                          NaOH                                                                              Gel Time                                                                           Brine                                      No.                                                                              Wt. %                                                                              Wt. %                                                                             Wt. % Wt. %   Wt. %                                                                             Hr.  TDS Wt. %                                  __________________________________________________________________________    1  0.2  0.01                                                                              1     1       0   16   12                                         2  0.2  0.01                                                                              1     1       0.002                                                                             14   12                                         3  0.2  0.01                                                                              1     1       0.004                                                                             12   12                                         4  0.2  0.01                                                                              1     1       0.006                                                                             10   12                                         5  0.2  0.01                                                                              1     1       0.01                                                                              8    12                                         6  0.2  0.01                                                                              0.5   0.5     0   NA   3.5                                        7  0.2  0.01                                                                              0.5   0.75    0   NA   3.5                                        8  0.2  0.01                                                                              1     1.5     0   NA   3.5                                        9  0.2  0.01                                                                              1.5   1.5     0   NA   3.5                                        10 0.2  0.01                                                                              1.5   0.75    0   NA   3.5                                        11 0.2  0.01                                                                              2     1       0   NA   3.5                                        12 0.2  0.01                                                                              2     1.5     0   NA   3.5                                        13 0.2  0.01                                                                              2     2       0   NA   3.5                                        __________________________________________________________________________     Examples 1-5 demonstrate the gel time control by adding NaOH.                 Examples 7-13 demonstrate the variation of resorcinol and formaldehyde        compositions.                                                                 Using the composition of Example 1 gels were also prepared in brines of 2     wt. %, 20 wt. %, 18 wt. %, 15 wt. % and 11 wt. %.                             Shear stability was demonstrated by using no. 1 example sheared for 2, 4,     6, 8 minutes with a Waring blender at top speed to liquids from which         final gel were formed at 250° F.                                       All examples gave firm gel at higher temperatures to yield the final gel.     .sup.1 Pfizer's Flocon 4800C Xanthan biopolymer.                         

In the practice of this invention, a xanthan gum biopolymer is used tomake a gel of the preferred composition above mentioned. The xanthan gumbiopolymer utilized comprises a product purchased from the Pfizer Co.and sold under the FLOCON® trademark. Xanthan gum, chromic nitrate,resorcinol, and formaldehyde are mixed into 6 wt. % seawater inaccordance with the preferred composition. By adding a sufficient amountof sodium hydroxide, a gel composition is obtained which forms a gelwithin a desired time. It also possesses the desired gel strength forinjecting into a more permeable zone of a formation. An initial gel isallowed to form at ambient temperatures. By ambient temperatures ismeant a temperature of from about 40° F. to about 100° F. Upon obtainingthe desired viscosity and strength, the gel is injected into at leastone more permeable zone of a formation. The behavior of a Xanthomonaspolysaccharide complex with trivalent metal ions is discussed in U.S.Pat. No. 4,574,887. This patent is incorporated herein by reference inits entirety.

The formation conditions into which the gel is injected may have a pHenvironment of from about 3.0 to about 10.0. The formation temperaturecan be from ambient to about 400° F. The final gel which forms canwithstand these temperatures and pH ranges. Indeed, a formationtemperature of about 100° to 400° F. allows the initial gel to from afirm, solid gel which is thermally and shear stable.

After the final thermally and shear stable gel has formed in a morepermeable zone of the formation an enhanced oil recovery method isinitiated in a less permeable zone so that hydrocarbonaceous fluids canbe removed therefrom. The enhanced oil recovery method which is utilizedcan include for example, a carbon dioxide method, a waterflood, or asteam flood.

A carbon dioxide oil recovery method which can be used in a lesspermeable zone after closing a more permeable zone of a formation withthe gel disclosed herein is disclosed in U.S. Pat. No. 4,523,642 whichissued to Venkatesan on June 18, 1985. This patent is herebyincorporated by reference herein in its entirety.

A waterflooding method which can be used in a zone of lesserpermeability after closing a more permeable zone of a formation withthis novel gel is described in U.S. Pat. No. 4,479,984 which issued toChen et al. This patent is hereby incorporated by reference herein inits entirety.

Steamflood methods which can be utilized when employing the novelpolymer gels described herein are detailed in U.S. Pat. Nos. 4,489,783and 3,918,521 issued to Shu and Snavely, respectively. These patents arehereby incorporated by reference herein.

The novel polymer gels described herein can also be used in conjunctionwith a cyclic carbon dioxide steam stimulation in a heavy oil recoverymethod to obtain greater sweep efficiency. Cyclic carbon dioxide steamstimulation can be commenced after plugging the more permeable zones ofthe reservoir with the aforedescribed novel polymer gels. A suitablemethod is described in U.S. Pat. No. 4,565,249 which issued to Pebdaniet al. This patent is hereby incorporated by reference in its entirety.Increased sweep efficiency can be obtained when the subject gels areused in combination with a carbon dioxide process by lowering the carbondioxide minimum miscibility pressure ("MMP") and recovering oil. Priorto commencement of the carbon dioxide method, the more permeable zonesare plugged with the disclosed novel polymer gels. Carbon dioxide MMP inan oil recovery method is described in U.S. Pat. No. 4,513,821 issued toShu. This patent is hereby incorporated by reference.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A process for closing pores in a more permeablezone of a formation comprising:(a) placing into an aqueous solution afirst composition sufficient to form ex-situ a size selective, shearthinning first gel which comprises(i) a xanthan biopolymer, and (ii) atransitional metal ion; (b) placing into said aqueous solution a secondcomposition sufficient to form thermally a second in-situ gel which issubstantially more resistant to formation conditions than said first gelwhich composition comprises(i) an aldehyde, and (ii) a phenoliccompound; (c) allowing said aqueous solution sufficient time to form theex-situ gel; and (d) injecting said aqueous solution containing said gelinto said permeable zone where it reheals, is heated by the formationand thereafter forms a solid gel substantially more resistant toformation conditions than said first gel.
 2. The process as recited inclaim 1 where after the gellable composition has formed a solid gel, anenhanced oil recovery method is initiated and hydrocarbonaceous fluidsare recovered from a less permeable zone of said formation.
 3. Theprocess as recited in claim 1 where after the gellable composition hasformed a solid gel, an enhanced oil recovery method selected from thegroup consisting of a water flood, a carbon dioxide recovery method, ora steam flood is initiated in a less permeable zone andhydrocarbonaceous fluids are recovered.
 4. The process as recited inclaim 1 where the formation has a temperature of up to about 400° F. 5.The process as recited in claim 1 where an alkali metal hydroxide isadded to said composition prior to injecting it into the formation in anamount of up to about three times the amount of the transitional metalcompound utilized.
 6. The process as recited in claim 1 where the pHenvironment of the formation wherein the composition is utilized is fromabout 3.0 to about 10.0.
 7. The process as recited in claim 1 where thewater comprises fresh water, salt water or brackish water.
 8. Theprocess as recited in claim 1 where an initial gel is formed withinabout 4 to about 12 hours at a temperature of about 40° to about 100° F.and the gel is injected into the formation where it forms a final gel ata temperature of about 100° to about 400° F.
 9. The process as recitedin claim 1 wherein said aqueous solution contains(i) about 0.1 to about1.0 wt. % of a xanthan biopolymer; (ii) about 20 to about 1,000 ppm oftransitional metal ions; (iii) about 0.25 to about 5 wt. % ofresorcinol; and (iv) about 0.5 to about 3 wt. % of formaldehyde.
 10. Theprocess as recited in claim 1 where the aqueous solution is about 3.5 toabout 22 wt. % synthetic seawater containing(i) about 0.2 to about 0.5wt. % of a xanthan biopolymer; (ii) about 100 ppm Cr⁺³ ions by weight;(iii) about 2.5 to about 1.0 wt. % resorcinol; (iv) about 0.5 wt. %formaldehyde; and (v) a sufficient amount of an inorganic acid or alkalimetal hydroxide to obtain a desired gel time and gel strength.
 11. Aprocess for closing pores in a more permeable zone of a formationcomprising:(a) placing into an aqueous solution a first compositionsufficient to form ex-situ a size selective, shear thinning first gelwhich comprises(i) about 0.1 to about 1.0 wt. % of a xanthan biopolymer,and (ii) about 20 to about 1,000 ppm of a transitional metal ion andmixtures of transitional metal ions; (b) placing into said aqueoussolution a second composition sufficient to form thermally a secondin-situ gel which is substantially more resistant to formationconditions than said first gel which composition comprises(i) about 0.25to about 5 wt. % of resorcinol, and (ii) about 0.5 to about 3 wt. % offormaldehyde; (c) allowing said aqueous solution sufficient time to formthe ex-situ gel; and (d) injecting said aqueous solution containing saidgel into said permeable zone where it reheals, is heated by theformation and thereafter forms a solid gel substantially more resistantto formation conditions than said first gel.
 12. The process as recitedin claim 11 where after the gellable composition has formed a solid gel,an enhanced oil recovery method selected from the group consisting of awater flood, a carbon dioxide recovery method, or a steam flood isinitiated in a less permeable zone and hydrocarbonaceous fluids arerecovered.
 13. The process as recited in claim 11, where the formationhas a temperature of up to about 400° F.
 14. The process as recited inclaim 11 where an alkali metal hydroxide is added to said compositionprior to injecting it into the formation in an amount of up to aboutthree times the amount of a transitional metal compound utilized. 15.The process as recited in claim 11 wherein the pH environment of theformation wherein the composition is utilized is from about 3.0 to about10.0.
 16. The process as recited in claim 11 where the water comprisesfresh water, salt water or brackish water.
 17. The process as recited inclaim 11 where an initial gel is formed within about 4 to about 12 hoursat a temperature of about 40° to about 100° F. and the gel is injectedinto the formation where it forms a final gel at a temperature of about100° to about 400° F.
 18. The process as recited in claim 11 where theaqueous solution is about 3.5 to about 22 wt. % synthetic seawatercontaining(i) about 0.2 to about 0.5 wt. % of a xanthan biopolymer; (ii)about 100 ppm Cr⁺³ ions by weight; (iii) about 0.5 to about 1.0 wt. %resorcinol; (iv) about 0.5 wt. % formaldehyde; and (v) a sufficientamount of an inorganic acid or alkali metal hydroxide to obtain adesired gel time and gel strength.